This invention is related to a polymer blend ("polyblend") 100 parts by weight (by wt) of which comprises from about 70:30 to about 30:70 parts of poly(vinyl chloride) ("PVC") and a copolyester ("CPE") having particular chemical and physical properties. The CPE is not referred to as a segmented copolyester ("SCPE") because this SCPE nomenclature refers to a copolyester in which there are readily distinguishable long chain segments and short chain segments. There are no chain segments of readily distinguishable length, whether long or short, in a CPE which consists essentially of all ester segments with no ether linkages, each segment containing an aryl dicarboxylic acid residue connected to either diol or polyester diol segments, the majority of which polyester diol segments have a number average molecular weight (Mn) less than 600.
In a CPE, a multiplicity of the ester diol segments are either derived from ester interchange with a polyester diol, or by polycondensation in those instances where a polyester diol with the desired repeating units is no readily available. Further, the terms CPE and SCPE serve to minimize the confusion resulting from referring to prior art segmented copolyesters containing long chain segments, and copolyesters used in the instant invention which CPEs were only derived from long chain polyester diols by ester interchange or by polycondensation.
The polyblend of the instant invention has essentially no plasticizing function typically obtained with prior art plasticizers, yet the polyblend has unexpected and desirable processing characteristics, especially when it is reinforced with glass fiber. The processability of my polyblend is particularly noteworthy because my CPE is non-elastomeric unlike prior art polyblends made with an elastomeric SCPE.
Articles formed from commercially available PVC, such as rigid extruded PVC pipe, and compression and injection molded articles of a great variety of shapes and sizes, have the common properties of relatively low heat distortion temperature (HDT), low impact resistance, low flexural strength and high susceptibility to failure due to repeated stress. One of the schemes in the prior art, to counter these negative qualities, is to provide a wide array of polyblends in which a relatively highly crystalline polyester having certain essential characteristics, is miscible. More commonly the polyblends included a polyester resin prepared from short chain aliphatic glycol and diacid monomers such as are disclosed in U.S. Pat. No. 3,574,789 which polyester has a crystalline melting point (m p) below about 170.degree. C., and is miscible in PVC, but which m p was simply too low to provide limited miscibility. It is self-evident that a useable polyester for the polyblend cannot have a m p above that of the processing temperature of the PVC if its (PVC's) degradation is to be avoided.
By "miscible" I refer to a solubility of the polyester in the PVC, or vice versa, such that the polyblend exhibits no discernible separation of amorphous phases, and a single glass transition temperature ("T.sub.g "). Among these polyblends are those disclosed in "A Study of Aromatic Polyester/Chlorinated Polymer Blends" by M. Aubin and R. E. Prud'homme, Polym. Engin.Sci. 24, 587 (1984).
Other references dealing with the problem of how to obtain complete or nearly complete miscibility characterized by the foregoing criteria include the following: "Applications of Polymer Blends: Emphasis on Recent Advances" by L. M. Robeson, Polym. Eng. Sci. 24, 587 (1984); "Miscibility of Semicrystalline Segmented Polyether Ester Copolymers in Poly(Vinyl Chloride)" by M. H.Lehr, Studies in Physical and Theoretical Chemistry, Vol 10 Elsevier Scientific Publishing Company (1978); "Improvement of the Impact Strength of a Blend of Poly(Vinyl Chloride) with Copolyester Thermoplastic Elastomer by Heat Treatment" by T. Nishi and T. K. Kwei, J. Appl. Polym. Sci., 20 1331-1337 (1976).
More particularly, this invention is directed to a polyblend prepared by mechanical mixing of its solid components which yields a mixture consisting of a matrix and a dispersed particle phase, any one of which contains within its physical boundary, two well-mixed polymers as distinct discernible phases. If a reinforcing filler such as glass fiber is added, the matrix includes a third phase. Each additional filler or immiscible additive constitutes an additional phase.
The recent interest in polyesters or copolyesters (together referred to hereinafter as "(co)polyesters" for brevity) which are miscible with chlorinated polymers such as PVC, chlorinated PVC ("CPVC"), chlorinated poly(ethylenes), and copolymers of vinylidene chloride, has centered upon the particular ester concentration of the (co)polyester which defines a "miscibility window" in which miscibility is found. Outside this window there is reputed to be "immiscibility", there purportedly being no intermediate condition or state. The CPEs used in the polyblend of this invention are neither immiscible nor inside the window of miscibility but have "limited miscibility" which is critical for polyester crystal formation during the melt cooling of the polyblend into shaped articles.
The term "polyblend" is used herein to connote a mixture of PVC and CPE which maintains a polyphase or multiple phase polymeric structure consisting essentially of PVC-rich and CPE-rich amorphous phases, and crystalline PVC and CPE phases, which structure satisfies the criteria defining "limited miscibility". A CPE having limited miscibility is one which has sufficient crystallinity so that, when blended with the PVC, the polyblend has distinct discernible phases characterized by a crystallinity in the range from 25% to about 55% based on CPE.
The essential criterion of limited miscibility which the CPE in the PVC/CPE polyblend of this invention meets, is specified in greater detail hereinbelow by giving (i) the composition of the polyblend and the characteristics of its components, (ii) the sample history as to method of preparation, (iii) the method and instruments used to determine limited miscibility, and (iv) the experimental results and conclusions.
The term "miscible" is preferred rather than, say, the term "compatible" because the latter term has been often misapplied and misconstrued despite studious and concerted efforts on the part of many to lend uniformity to its meaning (see, for instance, "The Concept of Compatibility in Polyblends" by Arthur J. Yu in the chapter titled "Multicomponent Polymer Systems", Encyclopedia of Polymer Science and Technology, inter alia).
In the prior art, the vague term "compatible" has been used as an equivalent for "miscible", as has been the term "plasticized with", irrespective of the amount used, whether the "plasticizer" is an internal plasticizer, that is, is chemically reactive with PVC which is thus plasticized, or miscible with it so that it does not bleed or bloom from it, or immiscible with it so that it does.
In all the foregoing instances in which a polyester is blended with PVC, there is a plasticizing effect attributable to the polyester used, as there is in the present invention, but only to the extent that there is a drop in modulus with increasing concentration of CPE. However, there is a characteristically small drop in modulus as a function of CPE concentration, and a distinctive lack of a straight line relationship which is typical of a plasticizer in a prior art polyblend as will be evident from FIG. 8 which will be explained more fully hereinafter. Particularly in comparison with commercially available prior art polyblends using Hytrel brand SCPEs, inter alia, this lack of typical plasticizing effect is attributable to my CPE being non-elastomeric while SCPEs are elastomeric. Thus, the type of plasticizing effect I obtain, and the conditions under which it comes into play have a critical effect on the processing characteristics of the polyblend and its physical and chemical properties, for example, its unique morphology, lack of substantial extensibility and good solvent resistance.
U.S. Reissue Pat. No. Re. 28,982 to Crawford et al. teaches a blend of PVC and a semicrystalline SCPE having long chain "soft" and short chain "hard" ester segments or units. The long chain units are derived from a dicarboxylic acid and a poly(alkylene oxide)glycol having a mol wt in the range 600-6000. The short chain units are derived from a dicarboxylic acid and a low mol wt diol. The blends so formed are elastomers marketed as the Hytrel.sup.R copolyesters. These SCPEs were used in the Lehr, supra, work in which he reported their partial miscibility in PVC which partial miscibility was enhanced after the primary particles of PVC began to break down, and, because the Hytrel SCPEs lacked the requisite chemical structure and crystallinity, he missed the critical significance and effect of obtaining at least 25% crystallinity in the polyblend, regardless of whether the phases he observed in his partially miscible blends were co-continuous, or, the morphology of the PVC primary particles in the blends was maintained.
Though it is noted that Crawford teaches that his SCPE may have as little as 5 wt % of the soft segments, even if these segments were derived from a polyester diol, the melting point of his SCPE would be above 220.degree. C., which is too high to be melt-blended with PVC. This melting point can be physcially measured, or calculated according to the equations given in the Jelinski article infra, and are found to be above the processing temperature of unplasticized PVC. The 5 wt% of the soft segments were specifically required to be derived from a polyester diol under conditions chosen to avoid "interchange during polymerization" (col 5, lines 1-11, '982 patent), that is, under conditions chosen to avoid breakup of the polyester.
It is also noted that, since all the segments in Crawford's SCPE are randomly joined, there is a small statistical chance that a long chain hard segment may be formed from the proliferation of short chain hard segments known to be formed. Even where, as in Hytrel 7246, the average hard block length is 22 units and 81% by wt of the SCPE is hard (see "C.sup.13 Nuclear Magnetic Resonance Studies of Solid Segmented Copolymers--I--Mobile Domains of a Polyester Thermopolastic Elastomer" by L. W. Jelinski, et al Macromolecules pg 583, Vol 14, No. 3, May-June 1981)., typical plasticization is clearly obtained. This was corroborated by the studies of Lehr, supra. There is no teaching in Crawford to suggest that long chain hard segments would be desirable for changing the morphology obtained by plasticization. Of course, a lack of plasticization was to be expected with increasing crystallinity, and an increase of melting point was expected to be obtained with a smaller percentage of soft segments, but the clear teaching of Crawford was to avoid those circumstances and nothing which enables one to make the necessary modifications to alter the SCPE to end up with a CPE.
Less than a year earlier, De Witt et al, in U.S. Pat. No. 3,686,361 had disclosed that poly(propylene terephthalate) ("PPT" for brevity) in a polyblend with PVC, improved processing characteristics and reduced the melt viscosity of the polyblend. Though it appeared that the mol wt of the PPT was not critical, they failed to realize that in amounts up to about 20%, the PPT had a plasticizing function without a substantial amount of additional conventional trioctyl trimellitate ("TOTM") plasticiser, but at higher percentages, so much TOTM was required that the hardness and forming temperatures of the polyblend were both severely depressed. They did not recognize that PPT, without short chain "soft" aliphatic or cycloaliphatic (together referred to as "(cyclo)aliphatic" for brevity) polyester segments interspersed in the copolyester chain, lacked the appropriate chemical structure and adequate balance between crystallinity and the morphology which dictates the limited miscibility of the copolyester in the PVC. They had no incentive to modify the high crystallinity of the PPT, and no reason to suggest that a relatively high proportion of "soft" short chain aliphatic polyester segments (10 to 30 parts by wt per 100 parts polyblend) of lesser crystallinity would adequately modify it, and thus provide the desired morphology.
It was never recognized that the essential key to forming a desirable polyblend of PVC with a polyester was to provide a CPE which had limited misciblity in the PVC, and that this key would be provided by a tailored CPE which was non-elastomeric and had a m p in the range from above 170.degree. C. to about 220.degree. C. Unlike the elastomeric SCPE of the '982 patent which has short chain hard PBT (say) segments, and long chain soft poly(alkylene oxide) ester glycol segments, the tailored CPE used in my invention has all ester segments more than 50% of which have a residue of a diol which has a Mn less than 600. Such a CPE becomes a specific and necessary component of the polyblend because it provides a balance between the crystallinity required and the limited miscibility of the CPE in the PVC. This balance is determined by the particular structure of the CPE, and, for this CPE, is peculiar to vinyl chloride homopolymer. Such limited miscibility of the CPE does not extend to copolymers of vinyl chloride.
Still more particularly, this invention is related to a PVC/CPE polyblend which is reinforced with glass or other inorganic fibers without substantially vitiating the strong affinity the two polymers show for each other despite the discernibly separate phases. This affinity is adduced by the small domain sizes of the polyblend, it being recognized that if "miscibility" is to mean "single homogeneous phase" such as is associated with nearly complete mixing of the molecules at the molecular level, as it does for simple liquids, only a few polyblends are found to be truly miscible, and the polyblend of this invention is not among them.